The large asteroid Vesta is a true relic of our Solar System's early history. Thanks to the recent detailed study by the Dawn spacecraft, we know it's an intact protoplanet, the type of object that collided and aggregated into the planets early in our Solar System's history. Vesta's southern regions are scarred by a huge impact basin consisting of overlapping craters that cover a significant fraction of the asteroid's surface. Now, a new simulation may show how Vesta survived its violent past, and how the giant impact scar and some other significant surface features came to be.

The simulation—created by M. Jutzi, E. Asphaug, P. Gillet, J.-A. Barrat, and W. Benz—began with a fully three-dimensional model of Vesta's interior and surface. The researchers then modeled two successive impacts at nearly the same location. The impacts in this model reworked the entire asteroid, and accurately reproduce many of its existing features. Differences in the geological content of the surface didn't match observations, however, so some of the initial assumptions of the model may need modification.

Many asteroids are rubble, debris held together by mutual gravitation. Vesta, on the other hand, has a differentiated interior, much like Earth: it has a solid core surrounded by some sort of mantle. That means Vesta is a protoplanet, one of many small objects that populated the early Solar System. According to the widely accepted models of planet formation, most of the protoplanets either shattered during collisions or merged to make planets.

As with other Solar System bodies, Vesta bears the scars of its history. The most substantial of these scars are the two large impact basins, Veneneia and Rheasilvia. (Both craters were named for virgins who served the goddess Vesta in Roman mythology—the vestals.) Rheasilvia formed about 1 billion years ago and is larger. Veneneia is smaller and formed at least 2 billion years ago; its presence was partly obscured by the later impact. Meteorites from Vesta, possibly ejected by the impacts forming Rheasilvia and Veneneia, have been found on Earth.

Rheasilvia has unusual spiral fissures that wind clockwise if you're looking "down" on the south pole of the asteroid. The crater is also marked by its raised central peak, which is the tallest mountain in the Solar System at 22 kilometers. (For comparison, Chomolungma—Mount Everest—on Earth rises a paltry 8.8 km above sea level.) Since Vesta is only about 573 km across at its widest, the impact creating Rheasilvia must have nearly shattered the asteroid to raise such a huge mountain.

The new simulation started with a spherical Vesta with a differentiated interior: crust, mantle, and core. In this sense, Vesta is much like a mini version of Earth and the other terrestrial worlds, including Earth's Moon. The researchers assumed the protoplanet was initially non-rotating, although this is almost certainly an oversimplification. Unlike most prior simulations, the new model was fully three-dimensional, rather than assuming symmetry around the axis of rotation.

The simulation then introduced two successive impacts. The first—the analog to Veneneia—kickstarted Vesta's rotation, aligned with the crater's central peak. The second impact, creating the equivalent to Rheasilvia, drastically altered the shape of the asteroid, creating something akin to the dinner-roll shape Vesta has today.

Because of the rotation introduced by the first impact, the edges of the simulated Rheasilvia collapsed in a twisted way. That resulted in spiral fissures, much like those seen in the real impact basin. (From a physics standpoint, that's the effect of the Coriolis force, the same force responsible for hurricanes and other cyclonic storms on Earth.)

The simulation did a remarkably good job of reproducing the general topographic features seen on the real Vesta. However, the geological composition of the surface in the model differed significantly from observations. The authors speculated that their initial assumptions about the composition of Vesta's crust were wrong. Perhaps the asteroid had experienced a kind of mixing of materials in the era before Veneneia formed, or the crust is largely made of volcanic materials (basalts).

In either case, the three-dimensional simulation that started with a spherical, non-rotating asteroid resulted in something very similar to the real Vesta observed by the Dawn spacecraft. These results reinforce our understanding of how impacts shaped the worlds of our early Solar System, including Earth.

One question my friends and I forgot to ask my astronomy teacher in college is how they determine the source of meteorites. For example, how do we know a given meteorite is from Vesta or Mars.Anyone got the time to explain that, or have a link to a good article on this?

One question my friends and I forgot to ask my astronomy teacher in college is how they determine the source of meteorites. For example, how do we know a given meteorite is from Vesta or Mars.Anyone got the time to explain that, or have a link to a good article on this?

Isotope ratios. Depending on where you are in the Solar System, there are different ratios of isotopes.

One question my friends and I forgot to ask my astronomy teacher in college is how they determine the source of meteorites. For example, how do we know a given meteorite is from Vesta or Mars.Anyone got the time to explain that, or have a link to a good article on this?

It's based on the makeup of the meteorite compared to the expected makeup of the object it is believed to have originated from. I think they now believe all HED meteorites originated from 4 Vesta. HED meteorites are relatively common as far as meteorites go, account for 5% of them. http://en.wikipedia.org/wiki/HED_meteorite